Chain branching agent and polyamide composition containing the same

ABSTRACT

The invention relates to a chain branching agent containing anhydride groups, which chain branching agent contains. (a) 5-75 mass % of a copolymer of at least an unsaturated dicarboxylic acid or a derivative thereof and a vinyl aromatic monomer; (b) 5-75 mass % of a copolymer of acrylonitrile and a vinyl aromatic monomer, (c) 0-80 mass % of an inert processing aid; and (d) 0-10 mass % customary additives; and in which (a) and (b) are miscible, the ratio (a)/(b) is from 1/3 to 3/1, and the total of (a)+(b)+(c)+(d) is 100%. Advantages of this chain branching agent in the preparation of, for instance, a polyamide composition with non-Newtonian melt flow behaviour include better reproducibility and strongly decreased formation of gel particles. The invention also relates to processes for the preparation of a chain branching agent and of a polyamide composition with non-Newtonian melt flow behaviour as well as to the use of such a polyamide composition for the manufacture of moulding articles.

The invention relates to a chain branching agent containing anhydridegroups. The invention also relates to a process for preparing the chainbranching agent. The invention also relates to a process for preparing apolyamide composition with non-Newtonian melt flow behaviour as well asa polyamide composition obtainable by the process and use thereof forthe manufacture of a moulded article.

Such a chain branching agent is known from EP-A-0495363. As chainbranching agent said publication describes a copolymer containinganhydride groups, preferably a copolymer of maleic anhydride and styrene(SMA), which chain branching agent is used for the preparation of apolyamide composition with increased melt viscosity that exhibitsnon-Newtonian melt flow behaviour. Such a polyamide composition isprepared by melt mixing a low-viscosity polyamide with the chainbranching agent.

Non-Newtonian melt flow behaviour is here understood to mean rheologicalbehaviour involving an increase in the melt viscosity of a moltenpolymer composition with decreasing shear rate. This phenomenon is alsoreferred to as structural viscosity. Anhydride groups are hereunderstood to mean anhydride groups or groups from which anhydridegroups can be formed under processing conditions, for example adicarboxylic acid group.

A drawback of the known chain branching agent from EP-A-0495363 is thatmelt mixing of chain branching agent and polyamide involves a high riskof the formation of gel particles. Another drawback of the known chainbranching agent is that the resulting polyamide composition exhibitsviscosity values that are poorly reproducible, with differences inviscosity occurring not only between different lots but also betweendifferent granules within a lot. These differences greatly interferewith stable processing of the polyamide composition by means of anextrusion or injection moulding technique to form moulded articles.

The object of the invention therefore is to provide a chain branchingagent containing anhydride groups that can be used for the preparationof a polyamide composition with non-Newtonian melt flow behaviour andthat does not exhibit said drawbacks or exhibits them to a substantiallydecreased extent.

Surprisingly, this aim is achieved according to the invention with achain branching agent that contains

-   (a) 5-75 mass % of a copolymer of at least an unsaturated    dicarboxylic acid or a derivative thereof and a vinyl aromatic    monomer;-   (b) 5-75 mass % of a copolymer of acrylonitrile and a vinyl aromatic    monomer;-   (c) 0-80 mass % of an inert processing aid; and-   (d) 0-10 mass % customary additives;    and in which (a) and (b) are miscible, the ratio (a)/(b) is from 1/3    to 3/1, and the total of (a)+(b)+(c)+(d) is 100%.

Use of the chain branching agent according to the invention yields amuch better reproducibility when preparing a polyamide composition withnon-Newtonian melt flow behaviour and allows much more accurate controlof the viscosity, while in addition hardly any problems are caused byformation of gel particles. Another advantage is that larger amounts ofchain branching agent can be added, so that there are fewer dosingproblems in the production of a polyamide composition.

Examples of suitable unsaturated dicarboxylic acids or derivativesthereof that can be used as monomer for component (a) are maleic acid oritaconic acid, or derivatives thereof, for example maleic anhydride(MA), N-phenyl maleinimide or itaconic anhydride. Dicarboxylic acidderivatives are here understood to be in particular anhydride or imidederivatives. Examples of suitable vinyl aromatic monomers are styrene,α-methylstyrene, or a styrene in which the aromatic ring contains ahalogen or alkyl substituent. At least is here understood to mean thatthe copolymer (a) may also contain a minor amount of one or more othermonomers. Preferably, however, (a) is a copolymer of maleic anhydrideand styrene (SMA). The MA content of (a) generally lies between 5 and 40mass %, preferably between 10 and 35, and more preferably between 20 and30 mass %. The advantage of a high MA content is that a higher degree ofbranching can be achieved through reaction with a thermoplastic polymercontaining amine groups, for example a polyamide. A further advantage ofthe chain branching agent according to the invention is that such a highMA content can be used without any undesirable degree of polymer chaincrosslinking and formation of insoluble particles (gel particles)occurring

Suitable vinyl aromatic monomers in component (b) are the same asdescribed for (a), with (b) preferably being a copolymer of styrene andacrylonitrile (SAN). The acrylonitrile (AN) content of (b) generallylies between 5 and 40 mass %, preferably between 10 and 35, and morepreferably between 20 and 30 mass %. The advantage of a higher ANcontent is a higher polarity of the copolymer, which improves thecompatibility with other polar polymers such as a polyamide.

As a rule, an SMA copolymer is miscible with a SAN copolymer if theratio between the MA content and the AN content of the respectivecopolymers is roughly between 1.6 and 0.6. Preferably the chainbranching agent according to the invention therefore contains SMA andSAN copolymers with a ratio between the MA content and the AN content ofbetween 1.6 and 0.6, more preferably between 1.2 and 0.8.

The ratio between the amounts of components (a) and (b) in the chainbranching agent according to the invention may in principle vary between3/1 and 1/3. Preferably the (a)/(b) ratio lies between 2/1 and 1/2. Thishas the advantage that a better dispersion of the chain branching agentaccording to the invention in polyamide is obtained.

The chain branching agent according to the invention preferably contains10-80 mass % of an inert processing aid. A thermoplastic polymer is anexample of a suitable processing aid. This has as advantages a muchsimpler mixing process for the preparation of a chain branching agentaccording to the invention and in addition dosing of the chain branchingagent is greatly improved and can be controlled much more accurately.This inert processing aid also serves as carrier material for thepreparation of a chain branching agent in the form of a SMA and SANconcentrate. Inert is understood to mean that the processing aid or thecarrier material does not react with the other components (a) and (b)and with a thermoplastic polymer containing amine groups to which thechain branching agent is later added, and neither does it interfere toany undesirable extent with the reaction between anhydride groups andamine groups. The inert processing aid is preferably a thermoplasticpolyolefin. This is understood to be a polymer or copolymer ofsubstantially at least one olefin, which may also incorporate minoramounts of other monomers. Suitable examples include various types ofpolyethylene, including low- and high-density polyethylene (LDPE, HDPE),ethylene/α-olefin copolymers such as plastomers, ethylene copolymerswith a vinyl monomer or an alkyl(meth)acrylate, such as for examplevinyl acetate or ethyl acrylate, and propylene homopolymer andcopolymers. The chain branching agent according to the inventionpreferably contains an LDPE as inert processing aid in view of its goodprocessability. In a special embodiment the chain branching agentaccording to the invention contains between 40 and 80 mass % of an inertprocessing aid.

As additives (d) the chain branching agent according to the inventionmay contain the customary stabilizers, antioxidants and the like. Thechain branching agent may also contain compounds that promote thedesired reaction between anhydride groups of the chain branching agentand amine groups of a thermoplastic polymer.

The invention also relates to a process for preparing a chain branchingagent according to the invention in which the components (a), (b), (c)and (d) are melt mixed and then formed into a granulate. This granulateis preferably granular, with dimensions comparable to those of plasticsgranulate known to one skilled in the art, but may also be in powderform. The advantages of a granular granulate are the good dosingproperties and good mixing with other granular components, such asplastics granulate.

The invention also relates to the use of a chain branching agentaccording to the invention as additive to increase the viscosity of athermoplastic polymer containing amine groups. Preferably thisthermoplastic polymer containing amine groups is a polyamide.

The invention also relates to a process for preparing a polyamidecomposition with non-Newtonian melt flow behaviour in which a polyamidehaving a lower viscosity and substantially Newtonian melt flow behaviouris melt mixed with a chain branching agent containing anhydride groupsaccording to the invention and optionally other additives.

Suitable polyamides are generally all thermoplastic polyamides, butpreferably semi-crystalline polyamides in view of their favourableprocessing and mechanical properties. Examples include polyamide 6 (PA6), PA 66, PA 46, PA 69, PA 610, PA 11, PA 12, PA MXD6, and copolymersand mixtures of such polyamides. Preferably the polyamide is a PA 6, PA66, PA 46 or a copolyamide thereof.

Preferably the polyamide used in the process according to the invention,in particular PA 6, has a relative solution viscosity (RSV) of 2.0 to3.5 (measured on a 1 mass % solution in 90% formic acid at 25° C.). Morepreferably, the polyamide has an RSV of 2.2 to 2.8 ₁ and most preferablyof 2.3 to 2.6.

Suitable polyamides generally have 0.1 to 1 amine group as end groupsper linear chain molecule, the amine groups content preferably being atleast 20 meq/kg, more preferably 30 meq/kg and most preferably 40meq/kg. The advantage of a higher amine groups content is a strongerincrease in viscosity and more pronounced non-Newtonian melt flowbehaviour as a result of reaction with anhydride groups in the chainbranching agent.

Preferably such an amount of chain branching agent is added in theprocess according to the invention that the content of component (a) inthe polyamide composition is 0.01-6 mass % relative to the polyamide,more preferably 0.05-3, and most preferably 0.1-1.5 mass %, the highercontents relating to an SMA with a low anhydride groups content and theother way round. A larger amount of chain branching agent and/or ahigher anhydride groups content in the chain branching agent generallyresults in a stronger increase in the viscosity and more pronouncednon-Newtonian melt flow behaviour of the polyamide composition.

The polyamide composition made using the process according to theinvention may also contain 0-60 mass % other additives. Preferably theseadditives are chosen so that they do not significantly interfere withthe desired reaction between amine groups and anhydride groups. Examplesof additives are stabilizers, antioxidants, colourants, release agentsand lubricants and the like, flame retardants, impact modifiers, andfillers and reinforcing agents. Preferably the polyamide compositioncontains at least a combination of a copper salt and an alkali halide,such as Cu/KI, as stabilizer. Examples of suitable impact modifiers arerubber-like polymers that not only contain a polar monomers such asolefins, but also polar or reactive monomers such as, among others,acrylates, epoxide, acid or anhydride containing monomers. Examplesinclude a copolymer of ethylene with (meth)acrylic acid or anethylene/propylene copolymer functionalized with anhydride groups. Theadvantage of such additives is that they do not only improve the impactstrength of the polyamide composition but also contribute to an increasein viscosity. Suitable fillers and reinforcing agents are mineralfillers such as clay, mica, talc, glass spheres and fibrous reinforcingagents such as glass fibres. As reinforcing agent the polyamidecomposition preferably contains 5-50 mass % glass fibres, relative tothe total composition, more preferably 10-45, and most preferably 15-40mass % glass fibres. Suitable glass fibres generally have a diameter of5-20 micron, preferably 8-15 micron, and are provided with a coatingsuitable for use in polyamide. The advantage of a polyamide compositionwith glass fibres is its increased strength and stiffness, particularlyalso at higher temperatures, which allows use at temperatures up toclose to the melting point of the polyamide in the polyamidecomposition. This is important especially for use in the automotivesector, and particularly in or near the engine compartment.

In a preferred embodiment of the process according to the invention,polyamide, chain branching agent according to the invention and otheradditives (components) are melt mixed using an extruder, preferably atwin-screw extruder, it being possible for components to be suppliedboth to the throat of the extruder and to the melt. The temperature atwhich the process is carried out depends on the melting point of thepolyamide used and generally lies between 200 and 350° C. Preferablysuch a throughput rate is chosen that the residence time in theextruder, combined with the temperature, is sufficient for the reactionbetween anhydride groups of the chain branching agent and amine groupsof the polyamide to take place virtually completely. The advantage of acomplete reaction is that the polyamide composition will not undergo anystrong changes in viscosity when it is later melt-processed using ashaping technique. Surprisingly, it has been found that this reactionproceeds virtually to completion with the chain branching agentaccording to the invention, whereas when use is made of a pure SMA aschain branching agent the reaction does not proceed to completion in thesame available time, while moreover no or hardly any gel particles areformed with the chain branching agent according to the invention.

After melt mixing and granulation followed by drying, the polyamidecomposition is suitable for further use. If desired the viscosity can beincreased further by means of a solid-phase post-condensation reaction.This involves exposure of the polyamide composition under reducedpressure for, for example, 1-50 hours to a temperature up to at mostabout 10° C. below the melting point of the polyamide in the polyamidecomposition until the desired viscosity level is reached. Such apost-condensation is applied in particular for the preparation of apolyamide composition to be processed by means of an extrusion blowmoulding technique, this yielding an even better melt strength.

In another embodiment of the process according to the invention thecomponents of a polyamide composition are first mixed in solid conditionand melt mixing takes place during a shaping step, for example during aan extrusion or injection moulding technique. The advantage comparedwith a process in which first a polyamide composition is prepared, whichis then shaped, is that one processing step is left out. This process ispossible because the chain branching agent according to the invention ismore readily and better dispersed in a polyamide and the chain branchingreaction takes place in a more controlled way than when using a state ofthe art chain branching agent.

In a special embodiment a polyamide composition as obtained in granularform using the process according to the invention is then mixed withgranules of a conventional polyamide composition having a lowerviscosity and optionally a different composition, and processed as suchas a granular mixture in a shaping step. This has the advantage thatvarious compositions whose rheological properties are suited to thedemands of a particular application can be obtained in a flexible andeconomic manner.

The invention also relates to a polyamide composition that can beobtained with the process according to the invention.

Compositions containing PA6, SAN and SMA are also known inter alia fromthe publication in J. Polym. Sci., Part B: Polym. Phys. 30(11), 1273-84(1992), but these are mixtures of, for example, 75 mass % PA6 with SAN,to which a smaller amount of SMA is added as compatibilizer, for exampleabout 2.5 mass %. It is the effects on the morphology of the mixturethat are important here, not the Theological behaviour.

The invention therefore also relates to a polyamide composition withnon-Newtonian melt flow behaviour containing a polyamide, such an amountof chain branching agent according to the invention that the content ofcomponent (a) is 0.01-6, preferably 0.05-3, and most preferably 0.1-1.5mass % (relative to the polyamide), and 0-60 mass % of other additives,of which preferably 1045 mass % glass fibres.

The invention also relates to the use of a polyamide compositionaccording to the invention for manufacturing a part or moulded articleby means of an injection moulding or extrusion technique. The higherviscosity and the non-Newtonian rheological behaviour of the polyamidecomposition according to the invention is advantageous in particularwhen shaping takes place by means of extrusion, for example into a rod,profile or tube, and in particular when a (hollow) moulded article ismanufactured using an extrusion blow moulding technique. Extrusion blowmoulding generally involves the manufacture of a tubular, still moltenpreform, the melt strength of the polyamide composition having to besuch that the preform does not stretch or change shape under its ownweight. Examples of extrusion blow moulding techniques that are used inpractice include coextrusion blow moulding, sequential extrusion blowmoulding and 3D extrusion blow moulding. The polyamide compositionaccording to the invention also offers advantages when processing takesplace by means of art injection moulding technique, because the highviscosity at a low shear rate to a large extent prevents the formationof burs caused by the flow of the molten polyamide composition betweenmould parts.

The invention also relates to a process for the preparation of a mouldedarticle in which at least two parts, for instance obtained by means ofan injection moulding or extrusion technique, are bonded together bymeans of a welding technique, with at least one of the partssubstantially consisting, at least at the location of a surface to bewelded, of a polyamide composition according to the invention.

A special embodiment is a process in which all parts substantiallyconsist of a polyamide composition according to the invention, so thatan optimum weld seam strength is obtained.

A welding technique is here understood to be a way of bonding severalparts together in which the plastic is heated to above its melting pointat the location of the contact areas to be bonded together, followingwhich the parts are pressed together and cooled. Heating of the contactareas can be effected in several ways, for instance by contacting thecontact areas with a heated metal plate, by heating the plastic usingultrasonic vibrations (ultrasonic welding), or by generating frictionheat by rotating the contact areas against each other (rotation welding)or rubbing them together (vibration welding) at a high speed. Preferablyvibration welding is used for joining parts as this makes it possible toproduce complex moulded articles. It has been found that if at least onepart is made from a polyamide composition according to the inventionthis results in a substantial improvement of the weld seam strength.

The invention also relates to a moulded article obtainable by means ofan injection moulding or extrusion technique and to a moulded articleconsisting of two or more parts bonded together by means of a weldingtechnique, which moulded parts contain a polyamide composition accordingto the invention.

In particular the invention relates to a moulded article for use in theautomotive industry, such as a convoluted tube, a bellows, a liquidcontainer, a component of the fuel system, an air-inlet manifold or anair duct.

The invention will now be further elucidated on the basis of thefollowing examples and comparative experiments.

Materials Used: SMA a styrene-maleic anhydride copolymer with an MAcontent of 28 mass % and an M_(w) of approx. 110,000 g/mol (typeStapron ® SZ28110, DSM, NL); SAN a styrene-acrylonitrile copolymer withan AN content of 28 mass %, MFI (220° C., 10 kg) 50 g/10 min (DSM, NL);LDPE a low-density polyethylene (type Lupolen ® 1810H, BASF, DE); EPImpact modifier, ethylene-propylene rubber modified with rubber approx.0.5 mass % MA, MFI (230° C., 2.16 kg) = 0.7 g/10 min, density 0.87 g/cm³(type Tafmer ® MP0610, Mitsui, JP) PA 6 a polyamide 6 with RSV = 2.5 (1mass % in formic acid, 25° C.), M_(n) approx. 15,000 g/mol, M_(w)approx. 29,000 g/mol, amine groups content 48 meq/kg (type Akulon ®C225, DSM, NL); GF standard polyamide glass fibres, fibre diameter 10 mmPA 6 a standard injection-moulding polyamide 6 type with 30 mass % GF30glass fibres, heat-stabilized, coloured black (type Akulon ® K224-HG6,DSM, NL);

EXAMPLE I

A 50/50 (m/m) mixture of SMA and SAN was prepared by feeding the twocomponents to a ZSK57 twin-screw extruder, with a temperature setting of230° C., at a speed of 200 r.p.m. Only with great difficulty could theexiting, clear melt be processed into granules, the strand formed beingvery brittle.

EXAMPLE II

On a ZSK57, with a temperature setting of 230° C. and at a speed of 200rpm, a mixture of SMA/SAN/LDPE was prepared, in a mass ratio of25/25/50. The throughput rate was 110 kg/hour, with 85% torque ascontrol value. The mixture was readily extruded and processed intogranules with regular dimensions.

EXAMPLES III AND IV

In an analogous manner to Example II SMA/SAN/LDPE mixtures with massratios of 37.5137.5/25 and 12.5/12.5/75 were prepared. Theprocessability of the sample with 25% LDPE was slightly lower than thatof the other samples.

EXAMPLE V AND COMPARATIVE EXPERIMENT A

The compositions indicated in Table 1 were prepared by feeding PA 6,chain branching agent according to Example I or SMA, 0.3 mass %copper/iodide stabilizer and 0.5 mass % black colour concentrate to thethroat of a ZSK57. Furthermore an impact modifier was added. Thismodifier consisted in part of a modified EP rubber and in part of LDPE.The temperature was set at 270° C., the throughput rate was about 50kg/hour and the melt was degassed by applying a negative pressure. Theextruded strands were chopped into granules and dried. Subsequently thematerials were post-condensed in the solid phase at a granuletemperature of 185° C. until the materials had an RSV of 5.1 (1 mass %in formic acid, 25° C.).

The rheological behaviour of the compositions was measured by means of aRheometrics RMS800 Dynamic-Mechanical Spectrometer (DMS) in the shearrate range of 0.1-100 rad/s at a temperature of 260° C. (parallel plategeometry, diameter=25 mm).

The results are summarized in Table 1. From the higher melt viscosityfound it can be deduced that a chain branching agent according to theinvention is more effective than pure SMA. This is also apparent fromthe higher viscosity ratio at low and high shear rates, respectively. Inaddition, a melt drawing test (melt temperature=260° C.) was conducted.The force needed to draw the strand in the melt is a measure of the meltstrength of the material. The melt drawability is sensitive toinhomogeneities in the melt such as gels.

The higher melt drawing force is indicative of the increasedeffectiveness of the branching reaction and the higher melt drawabilityindicates that the number of gel particles is reduced in comparison withthe comparative experiment. TABLE 1 unit Example V Comp. Exp. AComposition PA 6 mass % 88.5 88.9 Example I mass % 0.72 0 SMA mass % 00.36 EP rubber mass % 4.0 4.0 LDPE mass % 6.0 6.0 rheological propertiesRSV 5.1 5.1 η (0.1 rad/s) Pa · s 22,400 17,800 η (100 rad/s) Pa · s 16001550 η (0.1)/η (100) 14.0 11.5 Melt drawability 38 24 Melt drawing forcecN 8.5 7.9

EXAMPLES VI-VII AND COMPARATIVE EXPERIMENTS B-C

The compositions indicated in Table 2 were prepared by feeding PA 6,chain branching agent according to Example II or SMA, 0.25 mass %copper/iodide stabilizer, 0.3 mass % release agent and 2.0 mass % blackcolour concentrate to the throat of a ZSK30 twin-screw extruder and 30mass % glass fibre via a side feed to the melt. The temperature was setat 270° C., the throughput rate was about 10 kg/hour and the melt wasdegassed by applying a reduced pressure. The extruded strands werechopped into granules and dried. In comparative experiment A acommercial PA6 GF30 was used as reference material.

The rheological behaviour of the compositions was measured by means of aRheometrics RMS800 Dynamic-Mechanical Spectrometer (DMS) in the shearrate range of 0.1-100 rad/s at a temperature of 260° C. (parallel plategeometry, diameter=25 mm)

The compositions were processed into test bars on an Engel 80e injectionmoulding machine, with a temperature setting of 260° C. (melttemperature).

The tensile properties were determined in accordance with ISO 527-1, theimpact strength was measured in accordance with ISO 179/1eU andISO179/1eA.

The results are summarized in Table 2. From the higher melt viscosityfound it can be deduced that a chain branching agent according to theinvention is more effective than pure SMA. In addition, during theinjection moulding of Examples VI and VII a more stable process wasobserved, with smaller pressure variations with time, than incomparative experiment C. The increased viscosity is not found to haveany adverse effect on the injection moulding behaviour as such,supporting a non-Newtonian flow behaviour. The higher viscosity doesresult in higher toughness of the polyamide composition, judging fromthe somewhat higher elongation at break and (unnotched) impact strengthand the slight decrease in tensile strength. TABLE 2 Comp. Comp. unitEx. VI Ex. VII Exp. B Exp. C composition PA 6 mass % 66.3 65.7 PA 6 GF3067.0 GF mass % 30.0 30.0 30 Example II mass % 1.2 1.8 0 0 SMA mass % 0 00 0.45 tensile properties E-modulus MPa 9830 9690 9680 9440 Tensile MPa160 155 169 170 strength Elongation at % 4.1 4.3 3.7 4.0 break impactstrength Charpy kJ/m² 16 15 15 14 notched, 23° C. Ch. kJ/m² 92 86 82 99unnotched, 23° C. Charpy kJ/m² 9 9 9 9 notched, −30° C. Ch. kJ/m² 75 7368 78 unnotched, −30° C. rheological properties RSV 3.35 3.88 2.4 η (0.1rad/s) Pa.s 7450 15150 940 8900 η (100 rad/s) Pa.s 2230 2610 850 2150η(0.1)/η(100) 3.3 5.8 1.1 4.1

EXAMPLE VIII

Analogously to Examples III and IV a larger amount of a polyamidecomposition with 1.1 mass % of the chain branching agent from Example IIwas prepared using a ZSK58 extruder, temperature setting 250° C. andthroughput rate about 150 kg/hour at a speed of 250 rpm. The product hadan RSV of 3.43. By means of DMS 1 (0.1 rad/s) was measured to be 7415and η (100 rad/s)=1990, so that η(0.1)/η(100) was 3.4.

This polyamide composition and the reference material PA 6 GF30(comparative experiment B) were injection moulded into two articleswhich were subsequently joined by means of vibration welding to form anair-inlet manifold. In general, the resulting weld seam is the weakestpoint of such a part. Its strength was determined by means of burstingpressure tests under different conditions. The results obtained arepresented in Table 3. It can be concluded that the polyamide compositionaccording to the invention results in an appreciably higher weld seamstrength than the reference material, while in addition it meets therequired minimum values for this moulded article. TABLE 3 Unitrequirement Example VIII Comp. Exp. B Bursting pressure MPa ≧0.55 0.660.40 at 23° C. Bursting pressure MPa ≧0.45 0.60 0.34 at −30° C. Burstingpressure MPa ≧0.50 0.50 0.42 at 120° C.

1. Chain branching agent containing anhydride groups, characterized inthat the chain branching agent consists of (a) 5-75 mass % of acopolymer of at least an unsaturated dicarboxylic acid or a derivativethereof and a vinyl aromatic monomer; (b) 5-75 mass % of a copolymer ofacrylonitrile and a vinyl aromatic monomer; (c) 10-80 mass % of a homo-or copolymer of ethylene or propylene; and (d) 0-10 mass % customaryadditives; and in which (a) and (b) are miscible, the ratio (a)/(b) isfrom 1/3 to 3/1, and the total of (a)+(b)+(c)+(d) is 100%.
 2. Chainbranching agent according to claim 1, in which (a) is a copolymer ofmaleic anhydride and styrene and (b) is a copolymer of acrylonitrile andstyrene.
 3. Chain branching agent according to claim 1, which contains40-80 mass % (c).
 4. Chain branching agent according to claim 1, whichcontains 10-30 mass % (a) and 10-30 mass % (b).
 5. Chain branching agentaccording to claim 1, in which (a) and (b) contain 10-35 mass % maleicanhydride and acrylonitrile, respectively.
 6. Chain branching agentaccording to claim 1, in which (a) and (b) contain 20-30 mass % maleicanhydride and acrylonitrile, respectively.
 7. Chain branching agentaccording to claim 1, in which the ratio (a)/(b) lies between 2/1 and1/2.
 8. Chain branching agent according to claim 1, in which the inertprocessing aid is a thermoplastic polyolefin.
 9. Chain branching agentaccording to claim 8, in which the processing aid is a low-densitypolyethylene.
 10. Process for preparing a chain branching agentaccording to claim 1, in which the components (a)-(d) are melt mixed andthen formed into a granulate.
 11. Process for preparing a polyamidecomposition with non-Newtonian melt flow behaviour, which comprises meltmixing a polyamide having a lower viscosity and substantially Newtonianmelt flow behaviour is melt mixed with a chain branching agentcontaining anhydride groups according to claim 1, and optionally otheradditives.
 12. Process according to claim 11, wherein the polyamide is aPA 6, PA 66, PA 46 or a copolyamide thereof.
 13. Process according toclaim 11, wherein the polyamide has an amine groups content of at least20 meq/kg.
 14. Process according to claim 11, wherein an amount of chainbranching agent is used such that the polyamide composition has acontent of component (a) of 0.01-6 mass % (relative to the polyamide).15. Process according to claim 11, which further comprises melt-mixingat least 15-40 mass % glass fibre as additive.
 16. Polyamide compositionobtainable with a process according to claim
 11. 17. Polyamidecomposition with non-Newtonian melt flow behaviour containing apolyamide, such an amount of chain branching agent according to claim 1that the content of component (a) is 0.01-6 mass % (relative to thepolyamide), and 0-60 mass % other additives.
 18. A method for themanufacture of a part or moulded article comprising injection mouldingor extruding a polyamide composition according to claim
 16. 19. A methodaccording to claim 18, which comprises forming said part by extrusionblow moulding said polyamide composition.
 20. Process for thepreparation of a moulded article in which at least two parts are bondedtogether by means of a welding technique, wherein at least one of theparts substantially consists, at least at the location of a surface tobe welded, of said polyamide composition.
 21. Process according to claim20, in which all parts substantially consist of said polyamidecomposition.
 22. Process according to claim 20, wherein vibrationwelding is used as the welding technique.
 23. A molded articleobtainable with the process according to claim
 20. 24. A molded articleaccording to claim 23 for use in the automotive industry, such as aconvoluted tube, a bellows, a liquid container, a component of the fuelsystem, an air-inlet manifold or an air duct.